safety chain

plural safety chains

1. railwaysA normally slack chain for preventing excessive movement between a truck and a car body during a slue.
2. An auxiliary watchchain secured to the clothes, usually out of sight, to prevent stealing of the watch.
3. A chain of sheet metal links with an elongated hole through each broad end, made by doubling the first link on itself, slipping the next link through and doubling, and so on.
4. A security chain that allows a door to be opened to see who is on the other side without allowing them to enter.